I don't know the exact qualifications, and as I understand it's somewhat of a case by case basis, but you should at least be an advanced level student. The club race school isn't about making you faster, it's about you feeling comfortable with others cars in close proximity at speed, and driving on parts of the track that you didn't even know existed. So entering the school you should already be able to drive at a fast pace without thinking too hard about it because there's a lot you're going to be learning and immediately applying on track.